High-speed production line painting requires high-volume painting equipment, typical of which is the electrostatic sprayer. This sprayer routinely delivers a high volume of liquid per unit time, in the form of electrostatically charged droplets. Paint loss may be half that of other spray processes. The charge may be delivered typically as 50,000 to 100,000 volts. Droplets are able to "go around corners" and deposit on areas otherwise inaccessible. The charge on the substrate being sprayed is opposite to that of the droplets, and the substrate is grounded so as to bleed off any built-up electrostatic charge. If this charge is not bled off, the article will build up a charge the same as that of the spray particles, and will repel further particles. If the substrate is metal, there is generally no problem in achieving an effective ground. However, many substrates are non-conducting (insulative) plastics, which are best grounded by first coating them with a conductive pre-coat, which can then be connected to a grounding system. The thus coated substrate can then be routinely electrostatically spray-painted. Various conductive pre-coats or primers are commercially available for this purpose.
Items must be grounded to be electrostatically spray painted. Commonly used masking materials are unsuitable for masking during such painting because they are dielectrics, i.e. insulators, and thus cannot be grounded.
Electrostatic spray painting is described in the Kirk-Othmer Encyclopedia of Chemical Technology, 3d edition, volume 6, a Wiley-Interscience Publication, at pages 414-418, particularly at pages 417 and 418.
It is well known that static charge can be reduced by increasing the moisture content of the atmosphere, and thus the approach in the past has been to use an antistatic agent which will modify the inherently dielectric polymer to impart hydrophilic properties to it by providing functional groups that attract moisture to it. For instance, it is well known to apply external antistatic agents onto polymers by conventional coating or painting methods. Also, it is well known to employ internal antistatic agents which are volume dispersed by admixing in the polymer; i.e. incorporated into the polymer by compounding or extrusion prior to or during molding or film-forming operations, and which work by migrating to the polymer surface. This migration is colloquially referred to in the art of antistatic polymer technology as a "blooming" or "bleeding" effect. When the antistatic agent has not remained volume dispersed but instead has bloomed or bled to the surface, the mechanism for moisture attraction is the same as with the painted on external antistatic agents. The atmospheric moisture is attracted causing decay or dissipation of static charges, i.e. such films depend on ambient humidity. Accordingly a high rate of blooming is required. Such films can overbloom and lose their antistatic character if subjected to a 24 hour water shower or a prolonged heat exposure.
Many patents show quaternary amines (also referred to as quaternary ammonium compounds or salts) as antistatic agents. Examples are U.S. Pat. No. 3,324,091 to Sauides, U.S. Pat. Nos. 3,445,440, and 3,517,045 both to Susi and Arthen, U.S. Pat. No. 3,894,077 to Horikawa et al, U.S. Pat. No. 4,104,175 to Martinsson et al, U.S. Pat. No. 4,268,583 and T961009 both to Hendy, and U.S. Pat. No. 4,605,684 to Pcolinsky.
Also of interest is Japanese Published Patent Application Kokai No. 59-47243, Ito et al, assignors to Mitsui (published Mar. 16, 1984) which shows an electrically conductive resin composition comprising ethylene/alpha,beta unsaturated carboxylic acid copolymer and a tertiary alkanolamine.
More particularly, U.S. Pat. No. 3,933,779 issued Jan. 20, 1976 to Baron et al assignors to Fine Organics discloses an antistatic polymer blend comprising a synthetic polymer and an antistatically effective amount of a diethoxy-methyl-C.sub.4 to C.sub.18 alkyl-toluene sulfonate wherein the alkyl of 4 to 18 carbon atoms may be unsubstituted or substituted by halo or aryl. It is believed that Emerstat 6660 is a diethoxylated alkyl ammonium salt of the formula (A) (A')N[(CH.sub.2 CH.sub.2 O).sub.n H].sub.2.sup.+ A'OSO.sub.3.sup.- which is further described below.
Of general interest is U.S. Pat. No. 4,678,836 (July 7, 1987) McKinney et al assignors to Dow Chemical. It shows blends of linear low density polyethylene (LLDPE) and ethylene-acrylic acid (EAA).
Also, it is noted polyolefin loaded with carbon is commercially available.